1. Field of the Invention
The present invention relates to a cholesteric liquid crystalline material. More specifically, the present invention relates to a cholesteric liquid crystalline material usable in an electro-active element or electro-active lens for providing an optical power in an activated state of the electro-active element or electro-active lens and substantially no optical power in a deactivated state of the electro-active element or electro-active lens.
2. Description of the Related Art
An electro-active element is a device with an optical power that is alterable with the application of electrical energy. An electro-active element may be constructed from two substrates. An electro-active material may be disposed between the two substrates. The substrates may be shaped and sized to ensure that the electro-active material is contained within the substrates and cannot leak out. One or more electrodes may be disposed on each surface of the substrates that is in contact with the electro-active material. The electro-active element may include a controller to apply one or more voltages to each of the electrodes. The electro-active element may include a power supply operably connected to the controller. When electrical energy is applied to the electro-active material by means of the electrodes, the electro-active material's index of refraction may be altered thereby changing an optical property of the electro-active element, such as its focal length or diffraction efficiency, for example.
An electro-active element may be in optical communication with a base lens. The electro-active element may be embedded within or attached to a surface of the base lens to form an electro-active lens. A base lens may be an optical substrate or a conventional optical lens. The optical substrate may be a lens blank. A lens blank is a device made of optical material that may be shaped into a lens. A lens blank may be “finished”, meaning that the lens blank has both of its external surfaces shaped into refractive external surfaces. A finished lens blank has an optical power which may be any optical power including zero or plano optical power. A lens blank may be “semi-finished”, meaning that the lens blank has been shaped to have only one finished refractive external surface. A lens blank may be “unfinished”, meaning that neither external surface of the lens blank has been shaped into a refractive surface. An unfinished surface of an unfinished or semi-finished lens blank may be finished by means of a fabrication process known as free-forming or by more traditional surfacing and polishing. A finished lens blank has not had its peripheral edge shaped, edged, or modified to fit into an eyeglass frame.
An electro-active element may also be embedded within or attached to a surface of a conventional optical lens to form an electro-active lens. A conventional optical lens is any device or portion of a device that causes light to converge or diverge. A lens may be refractive or diffractive. A lens may be either concave, convex, or planar on one or both surfaces. A lens may be spherical, cylindrical, prismatic, or a combination thereof. A lens may be made of optical glass, plastic, thermoplastic resins, thermoset resins, a composite of glass and resin, or a composite of different optical grade resins or plastics. A lens may be referred to as an optical element, optical preform, optical wafer, finished lens blank, or optic. It should be pointed out that within the optical industry a device can be referred to as a lens even if it has zero optical power (known as plano or no optical power). The conventional optical lens may be a single focus lens or a multifocal lens such as a Progressive Addition Lens or a bifocal or trifocal lens.
The electro-active element may be located in the entire viewing area of the electro-active lens or in just a portion thereof. The electro-active element may be spaced from the peripheral edge of the optical substrate or conventional optical lens in order to allow the electro-active lens to be edged for spectacle frames. The electro-active element may be located near the top, middle or bottom portion of the lens. It should be noted that the electro-active element may be capable of focusing light on its own and does not need to be combined with an optical substrate or conventional optical lens.
An electro-active element may be capable of switching between a first optical power and a second optical power. The electro-active element may have the first optical power in a deactivated state and may have the second optical power in an activated state. The electro-active element may be in a deactivated state when one or more voltages applied to the electrodes of the electro-active element are below a first predetermined threshold. The electro-active element may be in an activated state when one or more voltages applied to the electrodes of the electro-active element are above a second predetermined threshold. Alternatively, the electro-active element may be capable of “tuning” its optical power such that the electro-active element is capable of providing a continuous, or substantially continuous, optical power change between the first optical power and the second optical power. In such an embodiment, the electro-active element may have the first optical power in a deactivated state and may have an optical power between a third optical power and the second optical power in an activated state, wherein the third optical power is above the first optical power by a predetermined amount.
Electro-active lenses may be used to correct for conventional or non-conventional errors of the eye. The correction may be created by the electro-active element, by the optical substrate or the conventional optical lens, or by a combination of the two. Conventional errors of the eye include lower order aberrations such as myopia, hyperopia, presbyopia, and astigmatism. Non-conventional errors of the eye include higher order aberrations that can be caused by ocular layer irregularities.
An electro-active element may include a liquid crystal. Liquid crystal is particularly well suited for electro-active lenses because it has an index of refraction that can be altered by generating an electric field across the liquid crystal. Lastly, the operating voltage of some commercially available liquid crystals for display applications is typically less than 5 volts. Furthermore, some liquid crystals possess bulk resistivities on the order of 1011 Ωcm or more, which reduces electrical power consumption.
The development of electro-active lens technology for ophthalmic applications places certain requirements on the technology that are critical to its success. One such requirement is that in the case of failure, the user of the electro-active lens must not be placed in a dangerous situation. Such a requirement is known as fail-safe operation. For example, a user may have electro-active spectacle lenses designed for the correction of presbyopia. Presbyopia is the loss of accommodation of the crystalline lens of the human eye that accompanies aging. This loss of accommodation first results in an inability to focus on near distance objects and later results in an inability to focus on intermediate distance objects. In the user's electro-active spectacle lenses, a conventional optic may correct for the user's far distance refractive error, if any. An electro-active element, when activated, may provide additional optical power to correct for the user's near and/or intermediate distance refractive error. When the user engages in far distance tasks such as driving, the electro-active element is deactivated thereby providing the user with proper far distance correction. When the user engages in near or intermediate distance tasks such as reading a book or looking at a computer screen, the electro-active lens is activated thereby providing the user with proper near distance correction. If the power source or the controller of the electro-active spectacle lenses should fail while the user is driving a car, it is vitally important that the electro-active element be capable of defaulting to a deactivated state so that the user is provided with proper far distance correction.
A second requirement for electro-active lens technology is that the electro-active lens must be insensitive to the polarization of the light it is meant to focus. Light is a transverse wave composed of electromagnetic field vectors which oscillate perpendicular to the light wave's direction of propagation. The path that a given field vector traces out in time (in most of optics only the electric field vector is considered) can be thought of as the polarization state (linear polarization for a linear path, circular for a circular path, etc.). The light emitted from most illumination sources (e.g. the sun, incandescent and fluorescent lamps) can be described as unpolarized or randomly polarized in which the direction of the electric field vector oscillates randomly with time. Despite the random oscillations of the electric field vector, at any given instant, the electric field vector can be broken into two orthogonal vector components, as can be done for perfectly polarized light. As is well known in the art, these vector components, by way of example only, may themselves be linearly polarized and orthogonal in a Cartesian sense, or circularly polarized, and orthogonal in that they propagate with right and left handed twists. In other instances, the electric field vector may be broken down into two orthogonal components which are elliptically polarized (of which circularly polarized is a unique form).
An effective electro-active lens technology must be insensitive to the polarization of light, i.e. it must be able to focus light having any polarization state. However, most liquid crystalline materials are birefringent (exhibit an anisotropy of the refractive index) and as such are highly polarization sensitive. Optical waves with different polarization states traveling through a birefringent medium may experience a different index of refraction depending upon their direction of travel. For liquid crystal display applications the issue of polarization sensitivity is addressed through the use of dichroic polarizing films to only allow linearly polarized light to enter the display. As mentioned above, randomly polarized light waves have an electric field vector which oscillates randomly with time. Malus' law states that the intensity of a light wave passing through a linear polarizer is proportional to cos2(θ), where θ is the angle between the light wave's polarization direction (the electric field vector's direction) and the linear polarizer's direction. Since the incoming light wave is randomly polarized, it contains all θ's at random. Therefore, the intensity of the light wave passing through the linear polarizer is the average of cos2(θ), which is 50%. Thus, using a polarizing film blocks 50% of randomly polarized incoming light making it an unattractive option for electro-active lenses since it is important to focus all incoming light.
Polarization sensitivity is addressed differently depending, in a large part, on the optical properties of the particular liquid crystal being utilized. A nematic liquid crystal is optically uniaxial and possesses a single axis of symmetry with respect to its optical properties. This axis is known as the “director”. The orientation of the director varies throughout the bulk of a nematic liquid crystal layer but through the use of alignment layers, can be made, on average, to point in a single direction, called the alignment direction. An alignment layer is a thin film, which, by way of example only, may be less than 100 nanometers thick and constructed from a polyimide material. The thin film is applied to the surface of substrates that comes into direct contact with liquid crystal. Prior to assembly of the electro-active element, the thin film is buffed in one direction (the alignment direction) with a cloth such as velvet. When the liquid crystal molecules come in contact with the buffed polyimide layer, the liquid crystal molecules preferentially lie in the plane of the substrate and are aligned in the direction in which the polyimide layer was rubbed (i.e., parallel to the surface of the substrate). Alternatively, the alignment layer may be constructed of a photosensitive material, which when exposed to linearly polarized UV light, yields the same result as when a buffed alignment layer is used. Thus, in the absence of an electric field, the director of the liquid crystal molecules points in the same direction as the alignment direction. In the presence of an electric field, the liquid crystal molecules orient in the direction of the electric field. In an electro-active element, the electric field is perpendicular to the alignment layer. Thus, if the electric field is strong enough, the director of the liquid crystal molecules will be perpendicular to the alignment direction. If the electric field is not strong enough, the director of the liquid crystal molecules will point in a direction somewhere between the alignment direction and perpendicular to the alignment direction.
Uniaxial optical materials possess two unique refractive indices, an ordinary refractive index (no) and an extra-ordinary refractive index (ne). The birefringence of the uniaxial optical material, Δn, is defined as Δn=ne−no. An optical wave traveling in a direction parallel to the liquid crystal's director will experience the ordinary refractive index (no) regardless of the optical wave's state of polarization as an optical wave is a transverse wave where the electric field (the portion of the wave that experiences the phase delay due to the index of refraction) oscillates in a direction perpendicular to the direction of propagation, as is well known in the art. However, an optical wave traveling along any other path will experience a refractive index between the values of no and ne; the exact value of the refractive index depends upon the optical wave's state of polarization and its path through the material. As mentioned above, if the uniaxial material is in contact with an alignment layer and no electric field is applied, the director of the uniaxial material will be in the same direction as the alignment direction. Therefore, an incoming light wave, which is traveling in an direction perpendicular to the layer of uniaxial material (and as such is polarized parallel to the director), will experience an index of refraction between the values of no and ne, depending on the polarization state of the incoming light wave. As the electric field is increased, the director of the material begins to point in a direction somewhere between the alignment direction and perpendicular to the alignment direction. An incoming light wave, which is traveling in a direction perpendicular to the layer of uniaxial material, is no longer polarized parallel to the material's director but is not perpendicular to the director either. Therefore, this light wave will also experience a differing index of refraction depending on its polarization state. If the electric field is strong enough, the director of the liquid crystal molecules will be perpendicular to the alignment direction. In this case, incoming light waves will be traveling in a direction parallel to the director and the applied electric field and the incoming light waves will be polarized in a direction perpendicular to the director and the applied electric field. In this scenario, light waves will experience the ordinary index of refraction (no) regardless of its polarization state.
An important feature for an electro-active lens is the ability to change the focusing power of the lens. Changing the focusing power of the lens is accomplished by altering the index of refraction of the electro-active element's electro-active material. However, changing the index of refraction of a uniaxial material to an intended index of refraction between no and ne is polarization sensitive. As mentioned above, all unpolarized light waves can be thought of as being linearly polarized, where the direction of polarization changes randomly in time. Thus, for the same reason that only 50% of randomly polarized light passes through a linear polarizer, only 50% of the incoming randomly polarized light will experience the intended index of refraction. Therefore, if an electro-active lens that operates in the presence of unpolarized ambient light is constructed from a single layer of nematic liquid crystal, it will only focus half of the incident light. This will result in a drastic and unacceptable drop in visual acuity for the wearer.
A polarization insensitive electro-active lens using nematic liquid crystal that focuses all incident light typically requires the use of two layers of liquid crystal, placed in series, and arranged such that the alignment directions of the layers are orthogonal to each other. As polarized light can be broken down into two orthogonal components, the orthogonal orientation of the alignment directions ensures that the orthogonal components of light of any polarization will be properly focused by either the first layer of liquid crystal or by the second layer of liquid crystal. The drawback to this approach is that the requirements for manufacturing and operating the lens (e.g., materials, electrical connections, and electrical power consumption) will be effectively doubled.
A third requirement for electro-active lens technology is that the electrical power consumption must be as small as possible. As mentioned above, using two layers of nematic liquid crystal is not an attractive option since the power requirements effectively double. Similarly, a single layer of polarization insensitive polymer dispersed liquid crystal, as described by Nishioka, et al. in U.S. Pat. No. 7,009,757, is undesirable since the operating voltages are prohibitive for spectacle lens applications.
A fourth requirement for electro-active lens technology is that the number of electrical connections per lens must be kept at a minimum. Ideally the number of electrical connections should be limited to two: one to provide a zero-voltage reference (commonly referred to as “ground”) and another to provide a zero-DC bias time varying voltage (i.e., the time averaged voltage is zero, such that there is no DC offset). While this is achievable with a single layer of polarization insensitive polymer dispersed liquid crystal, the voltages required for the operation of this liquid crystal prohibit the use of the technology in spectacle lenses.
Thus, there is a need for an electro-active lens technology which meets all four of the aforementioned requirements.